There is a trend toward widespread testing of blood and other body-fluids for analytes which are predictive of health conditions, such as risk of coronary disease. The amount of cholesterol present in the blood is one factor which is related to the risk of coronary artery disease.
Cholesterol circulates in the blood predominantly in the protein-bound form. The proteins which transport cholesterol are the lipoproteins, which are subdivided into three classes based on their density. The very-low density lipoproteins (VLDL) are triglyceride-rich lipoproteins which are synthesized in the liver and ultimately converted to low-density lipoproteins (LDL), which transport most of the plasma cholesterol in humans. The high-density lipoproteins (HDL) are lipoproteins which are involved in the catabolism of triglyceride-rich lipoproteins, and in the removal of cholesterol from peripheral tissues and transport to the liver. An inverse relationship between serum HDL levels and risk of coronary disease has been established. In particular, if the proportion of serum cholesterol associated with HDL is low, the risk of coronary disease is increased.
In view of the importance of relative serum cholesterol levels in risk assessment and management of atherogenic disease, considerable effort has been spent screening large populations of both normal and high-risk individuals for serum levels of HDL, LDL, as well as total cholesterol and triglycerides. The effectiveness of treatments of high-risk individuals has been monitored by regular testing of serum levels of cholesterol in the various lipoprotein compartments.
One method for specific HDL cholesterol testing is based on the selective precipitation of non-HDL lipoproteins in serum by polyanionic compounds, such as dextran sulfate, heparin, and phosphotungstate, in the presence of a group-II cation, such as Mg.sup.2+, Mn.sup.2+, and Ca.sup.2+. The specificity and degree of precipitation are dependent on a variety of factors, including the type and concentration of the polyanion/metal agent. In general, the order of precipitation of serum cholesterol particles, with increasing concentration of polyanion is VLDL, LDL, and HDL. HDL usually remains soluble at concentrations of heparin or dextran sulfate which completely precipitate lower density particles, although minor apoE species of HDL may be co-precipitated with lower density particles. By selective precipitation of lower density particles, HDL serum cholesterol levels can be determined.
In a typical lipid assay procedure, a small volume of blood is drawn and centrifuged to produce a clear plasma or serum sample fluid. The sample fluid is then aliquoted into several assay tubes, for determination of (a) total serum cholesterol, (b) triglycerides, and (c) HDL cholesterol. The HDL sample is precipitated, as above, and the lower density particles are removed by filtration or centrifugation prior to cholesterol detection. The samples are then reacted with an enzyme mix containing cholesterol esterase, cholesterol oxidase, peroxidase and a dye which can be oxidized to a distinctly colored product in the presence of H.sub.2 O.sub.2. The tubes may be read spectrophotometrically, and the desired total, HDL and LDL cholesterol values determined.
Despite the accuracy and reliability which can be achieved with the liquid-phase cholesterol assay just described, the assay has a number of limitations for use in widespread screening. First, the method uses a venous blood sample, requiring a trained technician to draw and fractionate the blood sample, and aliquot the treated blood to individual assay tubes. At least one of the sample tubes (for HDL determination) must be treated with a precipitating agent, and further processed to remove precipitated material. Although some of these procedures can be automated, analytical machines designed for this purpose are expensive and not widely available outside of large hospitals.
The invention disclosed in above-cited co-pending patent application provides an assay device which substantially overcomes many of the above-mentioned problems associated with liquid-assay procedures for measuring serum cholesterol levels. In one embodiment, the device is designed for measuring the concentration of HDL-associated cholesterol, in a blood sample also containing LDL and VLDL particles. The device includes a sieving matrix capable of separating soluble and precipitated lipoproteins as a fluid sample migrates through the matrix. A reservoir associated with the matrix is designed to release a precipitating agent, for selectively precipitating LDL and VLDL, as fluid sample is drawn into and through the matrix. This allows HDL separation from the precipitated lipoproteins, based on faster HDL migration through the sieving matrix. The fluid sample thus depleted of non-HDL lipoproteins is assayed for cholesterol.
In studies carried out in support of the above invention, it was discovered that exposure of blood fluid samples to the reagent used in precipitating the HDL and LDL particles also resulted in binding of HDL to the glass particles, resulting in spuriously low HDL cholesterol values. This problem is addressed and overcome in the present invention.